In the conventional Fourdrinier paper-manufacturing method, an aqueous pulp or suspension of cellulose fibres (known as “paper stock”) is placed onto the upper surface of a so-called endless web made of wire and/or a synthetic material. This wire web acts as a filter, which causes the cellulose fibres to be separated from the aqueous medium and form a so-called wet-paper sheet. During formation of this wet-paper sheet, the forming sieve acts as a filter which separates the aqueous medium from the cellulose fibres, as the aqueous medium passes through the openings in the sieve.
To accelerate the removal of the water, the filtering process is very often carried out with the additional action of a vacuum applied to the underside of the sieve, i.e. on the machine side. Once the paper sheet has left the forming end section it is transferred to a press section of the paper machine, at this point it is guided through the gap between a pair, or several pairs, of pressure rollers, over which is stretched another fabric: a so-called “press felt”. The pressure of the rollers acting on the paper sheet removes additional moisture, and is frequently enhanced by the presence of a “mat” layer within the press felt. After passing through the pressing section, the paper is sent to a drying section of the machine for further removal of moisture. After drying, the paper is ready for any secondary processing which may be undertaken and finally packing.
The sieves used in paper-machines are made available as endless webs, and are manufactured by one of two methods. According to the first method, the free ends of individual flat woven webs are connected together by a procedure known as “splicing”, and in so doing the endless web is formed. In flat-woven paper-machine sieves formed in this way, the warp threads run in the machine direction, and the filling or weft threads run in the cross direction. According to the second production technique, the paper-machine sieves are directly fashioned in the form of a continuous strip, by the so-called endless-web method. In this method, the warp threads run in the cross direction of the machine, with the weft threads in the machine direction. Within the relevant literature, abbreviations for these terms are commonly used, with MD standing for “machine direction” and CMD for “cross machine direction”.
Within the wet end section of a paper machine, it is extremely important to maintain the cellulose fibres in the suspension on the paper side of the sieve, and to avoid markings within the forming sheet. These markings can occur when individual cellulose fibres are oriented within the paper sheet, such that their ends coincide with interstices between the individual threads of the sieve. In general, an attempt is made to solve this problem by providing a permeable sieve structure which is possessed of a coplanar surface, and which further allows the paper fibres to form a bridge over adjacent threads in the fabric and not penetrate into the interstices between them. As used herein, “coplanar” means that the uppermost parts of the threads, those which define the paper-forming surface of the sieve and are termed floats or knuckles respectively, lie at substantially the same height, so as to present a surface which is substantially “planar”. Fine paper, such as that used for high-quality printing, carbonization, cigarettes, electrical capacitors, and other papers of similar quality, has previously been produced on very finely woven sieves, as these present the flattest surfaces.
In order to make the surface of the cloth as close to planar as possible, particularly in the case of forming sieves, the surfaces are very often ground down with fine-grain emery paper. Such grinding is intended to improve the topography of the paper, and lead to a better final surface. Unfortunately, by grinding the surface in this way, the thread floats and knuckles of a sieve become damaged; this can be seen in FIGS. 3 and 4 when compared with FIGS. 1 and 2. FIG. 1 shows a section of a forming sieve which has not been processed, that is the floats or knuckles have not been ground with emery paper. FIG. 2 shows a section of the sieve according to FIG. 1, but under greater magnification.
FIGS. 3 and 4 correspond to the photographs shown in FIGS. 1 and 2, with the exception that in the sieve according to FIGS. 3 and 4, the topography of the paper has been evened out by grinding down the floats or knuckles. Whilst this particular levelling procedure does not reduce the interior volume of the sieve, the thickness is slightly reduced. This has further disadvantageous side effects, in that the stability of the sieve is adversely affected as a result: primarily, the loss of material entails a lower sieve stiffness. Furthermore, it has been found that as a result of this mechanical intervention, the sieve suffers from increased abrasion and hence a shorter operating life. In the case of threads with small diameters, e.g. 0.11 mm to 0.13 mm, the grinding process reduces the cross section of the threads by 30-40%. Such severe mechanical alteration of the threads, and hence of the sieve, can be seen as the root cause of the reduction in sieve stiffness. This is a further problem, as current trends in the paper industry are moving increasingly towards even thinner sieves with correspondingly thinner thread diameters. With this progression, limits are being placed on the mechanical alterations possible in order to produce coplanar sieve surfaces.
To further elucidate the state of the art as shown in FIGS. 1 to 4, reference is also made to FIGS. 5 and 6 as well as 7 and 8. FIG. 5 shows the contact surface of a sieve according to FIGS. 1 and 2, the untreated sieve, wherein about 30% of the total surface comprises the contact surface of the sieve. FIG. 6 shows the “standard” shape of floats and knuckles present in an untreated sieve, according to FIGS. 1 and 2. FIGS. 7 and 8 detail the structure of a ground-down sieve, wherein removal of 0.02 mm from the protruding floats and knuckles, increases the contact surface of the sieve to about 34%. The float or knuckle shape after grinding is shown in FIG. 8.
An objective of the current invention, is the preparation of sieves that present a highly coplanar surface, at least on the paper side, but preferably on both the paper and machine sides. This is to be achieved, even for sieves that are considerably thinner than those disclosed in the art, and have correspondingly reduced thread diameters. In light of the various problems presented above, this objective is to be achieved in particular for so-called forming sieves, i.e. sieves intended for use in the wet end section of a paper machine.